Transflective liquid crystal display and method of fabricating the same

ABSTRACT

The invention relates to a transflective liquid crystal display device that has a high contrast ratio. The transflective liquid crystal panel includes a homogeneous liquid crystal such that the transflective liquid crystal display device will have an optical retardation when the voltage is applied. Therefore, in order to compensate the optical retardation caused by this liquid crystal, a thickness of the liquid crystal layer is adjusted. Moreover, a thickness of the retardation film is also adjusted. Accordingly, the complete dark state and the high contrast ratio are achieved in the liquid crystal display.

This application is a Divisional of U.S. patent application Ser. No.10/694,217, filed on Oct. 28, 2003 now U.S. Pat. No. 6,917,404 and whichis a Divisional of U.S. patent application Ser. No. 09/850,186 (now U.S.Pat. No. 6,657,689) filed on May 8, 2001 and claims the benefit of theKorean Application No. 2000-24471, filed May 8, 2000, all of which arehereby incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display, and moreparticularly, to a transflective liquid crystal display and method offabricating the same. Although the present invention is suitable for awide scope of applications, it is particularly suitable for a highcontrast ratio.

2. Discussion of the Related Art

In general, a liquid crystal display (LCD) is classified as atransmission type and a reflection type depending on implementing aninternal or external light source. The transmission type has a liquidcrystal display panel, which does not emit light itself, and has abacklight as a light-illuminating section.

The backlight is disposed at the rear or one side of the panel. Theamount of the light from the backlight that passes through the liquidcrystal panel is controlled by the liquid crystal panel in order toimplement an image display. In other words, the light from the backlightvaries and displays images according to the arrangement of the liquidcrystal molecules. However, the backlight of the transmission type LCDconsumes 50% or more of the total power consumed by the LCD device.Providing a backlight therefore increases power consumption.

In order to overcome the above problem, a reflection type LCD has beenselected for portable information apparatuses that are often usedoutdoors or carried with users. Such a reflection type LCD is providedwith a reflector formed on one of a pair of substrates. Thus, ambientlight is reflected from the surface of the reflector. The reflectiontype LCD using the reflection of ambient light is disadvantageous inthat a visibility of the display is extremely poor when surroundingenvironment is dark.

In order to overcome the above problems, a construction which realizesboth a transmissive mode display and a reflective mode display in oneliquid crystal display device has been proposed. This is so called atransflective liquid crystal display device. The transflective liquidcrystal display (LCD) device alternatively acts as a transmissive LCDdevice and a reflective LCD device. Due to the fact that a transflectiveLCD device can make use of both internal and external light sources, itcan be operated in bright ambient light as well as has a low powerconsumption.

FIG. 1 shows a typical transflective liquid crystal display (LCD) device11. The transflective LCD device 11 includes upper and lower substrates15 and 21 with an interposed liquid crystal 23. The upper and lowersubstrates 15 and 21 are sometimes respectively referred to as a colorfilter substrate and an array substrate.

On the surface facing into the lower substrate 21, the upper substrate15 includes a black matrix 16 and a color filter layer 17. The colorfilter layer 17 includes a matrix array of red (R), green (G), and blue(B) color filters that are formed, such that each color filter isdivided by the black matrix 16. The upper substrate 15 also includes acommon electrode 13 over the color filter layer 17 and the black matrix16.

On the surface facing into the upper substrate 15, the lower substrate21 includes an array of thin film transistors (designated as TFT “T” inFIG. 1) that act as switching devices. The array of thin filmtransistors is formed to correspond to the matrix of color filters. Aplurality of gate and data lines 25 and 27 are positioned and crossedover each other. A TFT is located near at each crossing portion of thegate and data lines 25 and 27. The lower substrate 21 also includes aplurality of pixel electrodes 19 in the area between the gate and datalines 25 and 27. Such an area is often referred to as pixel regions “P”,as shown in FIG. 1.

Each pixel electrode 19 includes a transparent portion 19 a and areflective portion 19 b. The transparent portion 19 a is usually formedof a transparent conductive material having good light transmissivity,such as indium tin oxide (ITO). Alternatively, the transparent portion19 a may be a hole. Moreover, a conductive metallic material having asuperior light reflectivity is used for the reflective portion 19 b.

FIG. 2, a schematic cross-sectional view of a transflective LCD device57 illustrating an operation of such devices. For convenience, the colorfilters 17 (shown in FIG. 1) are not shown in FIG. 2 because it does notaffect the polarization state of light. As shown in FIG. 2, thetransflective LCD device 57 includes lower and upper substrates 21 and15 and an liquid crystal layer 23 having optical anisotropy isinterposed therebetween.

The upper substrate 15 includes a common electrode 13 on its surfacefacing into the lower substrate 21. On the other surface of the uppersubstrate 15, an upper quarter wave plate (QWP) 45 (often referred to asa retardation film), which has a phase difference λ/4, and an upperpolarizer 55 are formed in series.

The lower substrate 21 includes a transparent electrode 50 on itssurface facing into the upper substrate 15. A passivation layer 48 and areflective electrode 19 b are formed in series on the transparentelectrode 50. The reflective electrode 19 b and the transparentelectrode 50 act together as a pixel electrode (the reference numeral 19of FIG. 1). The passivation layer 48 and the reflective electrode 19 balso have a transmitting hole 19 a.

Various configurations and structures may be implemented for the pixelelectrode in the transflective LCD device. However, the passivationlayer 48 should be formed between the transparent electrode 50 and thereflective electrode 19 b.

In order to form a pixel electrode, a transparent conductive materialsuch as ITO (indium tin oxide) or IZO (indium zinc oxide) is depositedon the lower substrate 21 and then patterned into the transparentelectrode 50.

Next, the passivation layer 48 is formed on the transparent electrode50. The conductive metallic material having superior reflectivity, suchas aluminum (Al) or the like, is deposited on the passivation layer 48and then patterned to form a reflective electrode 19 b. In thispatterning process, the transmitting hole 19 a as a transparent portionis formed at the central portion of the reflective electrode 19 b.Moreover the central portion of the passivation layer 48 correspondingto the hole 19 a is also patterned to expose the central portion of thetransparent electrode 50.

Accordingly, the transparent electrode 50 and the reflective electrode19 b serve as a pixel electrode. Moreover, this structure makesdifferent cell gaps “d₁” and “d₂” between the common electrode 13 andthe pixel electrode (the reflective electrode 19 b and the transparentelectrode 50). “d₁” denotes the first cell gap between the commonelectrode 13 and the reflective electrode 19 b while “d₂” denotes thesecond cell gap between the common electrode 13 and the transparentelectrode 50.

On the other surface of the lower substrate 21, a lower quarter waveplate 54 and a lower polarizer 52 are formed in series. Moreover, abacklight device 41 is arranged below the lower polarizer 52.

In a homogeneous liquid crystal or twisted nematic (TN), its moleculesare oriented in the vertical direction when a voltage is applied(V_(on)=5V) and used as a liquid crystal layer 23. When an opticalretardation “Δn·d₁” of a first cell gap is λ/4 (λ=550 nm) and a secondcell gap “d₂” is twice as large as the first cell gap “d₁” as describedby equations (1) and (2), an optical retardation “Δn·d₂” of the secondcell gap “d₂” is shown in equation (3).Δn·d ₁=λ/4  (1)d₂≅2d₁  (2)∴Δn·d ₂≅λ/2  (3)

In the above equations, Δn is birefringence, d₁ denotes the first cellgap between the reflective electrode and the common electrode, d₂denotes the second cell gap between the transparent electrode and thecommon electrode. λ is the wavelength of the light, and λ/4 is a phaseshift value of the light when the light passes through a reflectiveportion of the liquid crystal layer 23 between the common electrode 13and the reflective electrode 19 b at once. λ/2 is a phase shift value ofthe light when the light passes through a transparent portion of theliquid crystal layer between the common electrode 13 and the transparentelectrode 50 at once.

Accordingly, the optical retardation “Δn·d₂” of the second cell gap“d₂”, as shown by equation (3), is λ/2 (λ=550 nm). In the reflectivemode, the ambient light passes through the liquid crystal layer 23twice, i.e., as the ambient light is reflected by the reflectiveelectrode 19 b.

As mentioned above, since different cell gaps (the transparent portionand the reflective portion) are formed in the liquid crystal layer 23,there is no difference in the optical retardation of light passing boththrough the transparent portion and through the reflective portion.

FIG. 3 shows a liquid crystal orientation in cases that the voltage isapplied and not applied. As shown, molecules of the liquid crystal layer23 are arranged in the horizontal direction along the upper and lowersubstrates 13 and 21 when the voltage is not applied. On the other hand,the molecules are arranged in the vertical direction perpendicular tothe upper and lower substrates 13 and 21 when the voltage is applied.However, in the ON-state, the molecules close to the upper and lowersubstrate 13 and 21 are not oriented properly because of an anchoringenergy generated between the liquid crystal molecules and eachsubstrate.

Therefore, the liquid crystal layer 23 derives characteristics ofbirefringence because the liquid crystal molecules are not properlyoriented. Namely, a residual optical phase retardation can exist becauseof unchanged orientation or alignment of some of the liquid crystalmolecules that are close to the upper and lower substrates 13 and 21.These cause the light leakage in a dark state of the LCD device.

In general, in case of the TN liquid crystal that has a twisted angle of90°, molecules detached from the upper and lower substrates are mostlyarranged perpendicular to the pair of substrates when the voltage isapplied since these molecules are not affected from the anchoringenergy. Moreover, the molecules close to the pair of substrates are notarranged in the vertical direction. Thus, the orientation direction ofthe TN liquid crystal molecules close to the upper substrate arearranged perpendicular to that of the molecules close to the lowersubstrate. As a result, an optical effect of the TN liquid crystal isoffset each other.

However, in case of the homogeneous liquid crystal that has a twistedangle of 0° as shown in FIG. 3, these molecules close to the upper andlower substrates 13 and 21 affect the optical effect of the liquidcrystal layer 23. This is because an orientation direction of themolecules located close to the upper substrate 13 are parallel to thatof the molecules around the lower substrate 23.

Therefore, a light leakage occurs in the dark state of the LCD devicewhen the upper retardation film (the reference numeral 45 of FIG. 2) andthe lower retardation film (the reference numeral 54 of FIG. 2) have thesame phase difference value. In addition, a contrast ratio of thetransflective LCD device is deteriorated by the light leakage.

FIG. 4 is a simplified cross-sectional view in order to calculate aphase retardation value of the above-mentioned homogeneous liquidcrystal. As shown, the upper polarizer 55 and the lower polarizer 52 arefacing into each other. The liquid crystal layer 23 that has the opticalretardation λ/2 is interposed between the pair of polarizers 55 and 52.Thereafter, a transmittance is measured by a simulator such as an LCDmaster.

FIG. 5 is a graph illustrating a transmittance when a voltage is appliedto the transflective LCD device of FIG. 4. When the voltage is applied,i.e., the TFT is turned ON, the transmittance should be ideally zero(i.e., T=0). However, the transmittance results in 0.038 (i.e., T=0.038)in experiment. Moreover, the transmittance can be calculated by thefollowing equation (4).

$\begin{matrix}{T = {{Sin}^{2}2\phi\;{{Sin}^{2}\left\lbrack \frac{{\pi \cdot \Delta}\;{n \cdot d}}{\lambda} \right\rbrack}}} & (4)\end{matrix}$

In equation (4), “T” denotes a transmittance, “Δn·d” denotes an opticalretardation, λ denotes a wavelength of light, φ denotes an angle betweena tranmissive axis of the polarizer and an optical axis of the liquidcrystal layer. From the above equation (4), the optical retardation“Δn·d” is 34 nm (i.e., Δn·d=34 nm), when λ is 550 nm and φ is 45degrees.

Accordingly, the light passing through the liquid crystal layer, whichincludes a homogeneous liquid crystal, has an optical retardation when avoltage is applied to the related art transflective LCD device. Thus, acomplete dark state can not be achieved because of the light leakagedescribed above.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a transflective liquidcrystal display and a method of fabricating the same that substantiallyobviate one or more of the problems due to limitations and disadvantagesof the related art.

An object of the invention is to provide a transflective LCD display anda method of fabricating the same achieving a high contrast ratio.

Additional features and advantages of the invention will be set forth inthe description that follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims hereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, atransflective liquid crystal display includes upper and lower substratesfacing into and spaced apart from each other, wherein the upper andlower substrates include a plurality of pixel regions that displayimages, a liquid crystal layer interposed between the upper and lowersubstrates, wherein the liquid crystal layer has a first adjustedthickness to compensate an residual optical retardation of incidentlight caused by anchored liquid crystals near an alignment layer when amaximum operation voltage is applied, an upper quarter wave plate (QWP)on the upper substrate, wherein the upper quarter wave plate has asecond adjusted thickness to compensate the residual optical retardationcaused by the liquid crystal layer when the maximum operation voltage isapplied, an upper polarizer on the upper quarter wave plate, atransparent common electrode on a surface of the upper substrate facinginto the lower substrate, a pixel electrode over a first surface of thelower substrate, wherein the pixel electrode corresponds to each pixelregion, and the pixel electrode is divided into transparent andreflective portions, a lower quarter wave plate (QWP) on a secondsurface of the lower substrate, a lower polarizer below the lowerquarter wave plate, and a backlight device arranged to be adjacent tothe lower polarizer.

The liquid crystal layer has a thickness of “d” and a transmittance of“T” when a maximu voltage for operating is applied, wherein an adjustedthickness of liquid crystal layer is calculated using the followingequation:

${T = {{Sin}^{2}2\phi\;{{Sin}^{2}\left\lbrack \frac{{\pi \cdot \Delta}\;{n \cdot d_{1}}}{\lambda} \right\rbrack}}},$where, T is equals to the value of the transmittance, φ is an anglebetween an optical axis of the liquid crystal layer and a transmissiveaxis of the polarizer, Δn is a birefringence of the liquid crystallayer, and wherein d₁ is calculated from the above equation, and theliquid crystal layer then has the adjusted thickness “d+d₁” forcompensating the optical retardation. A value of φ mentioned above is 45degrees.

The upper QWP has a thickness of “d” and a transmittance of “T”, whereinan adjusted thickness of the upper QWP for compensation is calculatedusing the following equation:

${T = {{Sin}^{2}2\phi\;{{Sin}^{2}\left\lbrack \frac{{\pi \cdot \Delta}\;{n \cdot d_{2}}}{\lambda} \right\rbrack}}},$

where, T is equals to the value of the transmittance, φ is an anglebetween a slow axis of the upper QWP and a transmissive axis of thepolarizer, Δn is a birefringence of the upper QWP, and wherein d₂ iscalculated from the above equation and the upper QWP then has theadjusted thickness “d+d₂” for compensating the optical retardation. Avalue of φ mentioned above is 45 degrees. In accordance with the purposeof the invention, in another aspect, the principles of the presentinvention provide a transflective liquid crystal display device,including: upper and lower substrates facing and spaced apart from eachother, wherein the upper and lower substrates include a plurality ofpixel region that display images; a liquid crystal layer interposedbetween the upper and lower substrates, wherein the liquid crystal layerhas an adjusted thickness of compensating a residual optical retardationof incident light caused by anchored liquid crystals near an alignmentlayer when a maximum voltage for operating is applied; a first upperretardation film over the upper substrate; a second upper retardationfilm between the first upper retardation film and the upper substrate,wherein the second upper retardation film has an adjusted thickness ofcompensating an optical retardation caused by the liquid crystal layer;an upper polarizer on the first upper retardation film; a transparentcommon electrode on a surface of the upper substrate facing the lowersubstrate; a transparent electrode on a surface of the lower substratefacing the upper substrate; a pixel electrode over the lower substrate,wherein the pixel electrode corresponds to each pixel region, andwherein the pixel electrode is divided into transparent and reflectiveportions; a second lower retardation film on the other surface of thelower substrate, wherein the second lower retardation film has anadjusted thickness of compensating a residual optical retardation causedby the liquid crystal layer when a maximum voltage for operating isapplied; a first lower retardation film under the second lowerretardation film; a lower polarizer under the first lower retardationfilm; and a backlight device arranged adjacent to the lower polarizer.

The transparent portion of the pixel electrode includes a transparentelectrode, and wherein the transparent electrode is disposed on asurface of the lower substrate facing the upper substrate. Thetransflective liquid crystal display device further comprises apassivation layer on the transparent electrode, the passivation layerhaving a transmitting hole in its central portion. Moreover, thereflective portion of the pixel electrode includes a reflectiveelectrode, which is disposed on the passivation layer and has thetransmitting hole in its central portion.

The transflective liquid crystal display device further includes apassivation layer on the transparent electrode, and the passivationlayer has a transmitting hole in its central portion. Moreover, thetransflective liquid crystal display device further includes areflective electrode on the passivation layer, and the reflectiveelectrode has the transmitting hole in its central portion.

The liquid crystal layer has a thickness of “d” in a reflective portionin order for an optical retardation of λ/4, and a thickness of “2d” in atransmissive portion in order ofr an optical retardation of λ/2, whereinthe liquid crystal layer has a transmittance of “T” when a maximumvoltage for operating is applied, wherein an adjusted thickness of theliquid crystal layer is calculated using the following equation:

${T = {{Sin}^{2}2\phi\;{{Sin}^{2}\left\lbrack \frac{{\pi \cdot \Delta}\;{n \cdot d_{*}}}{\lambda} \right\rbrack}}},$where, T is equals to the value of the transmittance, φ is an anglebetween an optical axis of the liquid crystal layer and a transmissiveaxis of the polarizer, Δn is a birefringence of the liquid crystallayer, and d* is a thickness of the liquid crystal layer, wherein d* isd₁ or d₂, wherein d₁ and d₂ are calculated from the above equation,where, d₁ is a first auxiliary thickness of the liquid crystal layerwhen the residual optical retardation of the light is γ in thereflective portion, d₂ is a second auxiliary thickness of the liquidcrystal layer when the residual optical retardation of the light is ω inthe transmissive portion, and then the phase difference between thetransmissive and reflective portions is δ=ω−γ, and wherein thereflective portion of the liquid crystal layer has the adjustedthickness of “d+d₁” and the transmissive portion of the liquid crystaldisplay has the adjusted thickness of “2d+d₂” for compensating theoptical retardation. A value of φ mentioned above is 45 degrees.

The second upper retardation film has a thickness of “d₄” and atransmittance of“T”, wherein an adjusted thickness of the secondretardation film is calculated using the following equation:

${T = {{Sin}^{2}2\phi\;{{Sin}^{2}\left\lbrack \frac{{\pi \cdot \Delta}\;{n \cdot d_{*}}}{\lambda} \right\rbrack}}},$where, T is equals to the value of the transmittance, φ is an anglebetween a slow axis of the retardation film and a transmissive axis ofthe polarizer, Δn is a birefringence of the retardation film, and d* isa thickness of the liquid crystal layer, wherein d* is d₁, d₂ or d₃,wherein d₁, d₂ and d₃ are calculated from the above equation and thefollowing equation:d _(2(ω)) =d _(1(γ)) +d _(3(δ)),where, d₁ is a first auxiliary thickness of the liquid crystal layerwhen the residual optical retardaion of the light is γ in the reflectiveportion, d₂ is a second auxiliary thickness of the liquid crystal layerwhen the residual optical retardaion of the light is ω in thetransmissive portion, and then the phase difference between thetransmissive and reflective portions is δ=ω−γ, and wherein the secondupper retardation film has the thickness of “d₄+d_(1(γ))” forcompensating the optical retardation. A value of φ mentioned above is 45degrees.

The second lower retardation film has the thickness of “d₄−d_(3(δ))” forcompensating the optical retardation. The first upper and lowerretardation films are beneficially half wave plates (HWPs) and thesecond upper and lower retardation films are beneficially quarter waveplates (QWPs). The transmissive axis of the lower polarizer isperpendicular to that of the upper polarizer. The slow axis of the firstupper retardation film is perpendicular to that of the first lowerretardation film. The slow axis of the second upper retardation film isperpendicular to that of the second lower retardation film. The opticalaxis of the liquid crystal layer is parallel with the slow axis of thesecond lower retardation film.

In another aspect of the present invention, a transflective liquidcrystal display device includes upper and lower substrates facing andspaced apart from each other, wherein the upper and lower substratesinclude a plurality of pixel region that display images; an upperquarter wave plate (QWP) on the upper substrate; an upper polarizer onthe upper quarter wave plate; a lower quarter wave plate (QWP) under thelower substrate; a lower polarizer under the lower quarter wave plate; abacklight device arranged adjacent to the lower polarizer; a liquidcrystal layer interposed between the upper and lower substrates; atransparent common electrode on a surface of the upper substrate facingthe lower substrate; an upper alignment layer between the transparentcommon electrode and the liquid crystal layer; a pixel electrode overthe lower substrate, wherein the pixel electrode corresponds to eachpixel region, and wherein the pixel electrode is divided intotransparent and reflective portions; and a lower alignment layer betweenthe pixel electrode and the liquid crystal layer; wherein a transmissiveaxis of the upper polarizer is perpendicular to a transmissive axis ofthe lower polarizer; wherein a slow axis of the upper QWP isperpendicular to a slow axis of the lower QWP; wherein the slow axis ofthe upper QWP forms an angle of 45° with the transmissive axis of theupper polarizer; wherein an optical retardation of the upper QWP isλ/4+α; wherein a ranges from zero to 100 nm; and wherein the slow axisof the lower QWP is parallel with an orientation direction of the liquidcrystal display layer.

An optical retardation of the liquid crystal layer is λ/4+α. The opticalretardation of the liquid crystal layer is different betweentransmissive and reflective portions, wherein the optical retardation isλ/4+α in the reflective portion, and wherein the optical retardation isλ/2+β in the transmissive portion, and wherein β ranges from zero to 100nm. Here, an optimum value of α for adjusting the optical retardationranges from zero to 50 nm and an optimum value of β for adjusting theoptical retardation ranges from zero 50 nm.

In another aspect of the present invention, a transflective liquidcrystal display includes upper and lower substrates facing and spacedapart from each other, wherein the upper and lower substrates include aplurality of pixel region that display images; an upper quarter waveplate (QWP) on the upper substrate; an upper half wave plate (HWP) onthe upper QWP; an upper polarizer on the upper HWP; a lower quarter waveplate (QWP) under the lower substrate; a lower half wave plate (HWP)under the lower QWP; a lower polarizer under the lower HWP; a backlightdevice arranged adjacent to the lower polarizer; a liquid crystal layerinterposed between the upper and lower substrates; a transparent commonelectrode on a surface of the upper substrate facing the lowersubstrate; an upper alignment layer between the transparent commonelectrode and the liquid crystal layer; a pixel electrode over the lowersubstrate, wherein the pixel electrode corresponds to each pixel region,and wherein the pixel electrode is divided into transparent andreflective portions; and a lower alignment layer between the pixelelectrode and the liquid crystal layer; wherein a transmissive axis ofthe upper polarizer is perpendicular to a transmissive axis of the lowerpolarizer; wherein a slow axis of the upper QWP is perpendicular to aslow axis of the lower QWP; wherein a slow axis of the upper HWP isperpendicular to a slow axis of the lower HWP; wherein an opticalretardation of the upper QWP is λ/4+α; wherein a ranges from zero to 100nm; wherein the slow axis of the lower QWP is parallel with anorientation direction of the liquid crystal display layer; wherein anoptical retardation of the lower QWP is λ/4−β; and wherein β ranges fromzero to 100 nm.

An optical retardation of the liquid crystal layer is different betweentransmissive and reflective portions, wherein the optical retardation isλ/4+αin the reflective portion, and wherein the optical retardation isλ/2+α+βin the transmissive portion. An optimum value of α ranges fromzero to 50 nm for adjusting the optical retardation and an optimum valueof β ranges from zero to 50 nm for adjusting the optical retardation.

The slow axis of the upper HWP forms an angle of θ with the transmissiveaxis of the upper polarizer. The slow axis of the upper QWP forms anangle of 2θ+45° with the transmissive axis of the upper polarizer.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiments of the invention andtogether with the description serve to explain the principle of theinvention.

In the drawings:

FIG. 1 shows a typical transflective liquid crystal display (LCD)device;

FIG. 2 shows a schematic cross-sectional view of a transflective LCDdevice illustrating an operation of such devices;

FIG. 3 is a schematic view illustrating a liquid crystal orientationwhen a voltage is applied or not applied to the crystal;

FIG. 4 is a simplified cross-sectional view in order to calculate aphase retardation value of the homogeneous liquid crystal;

FIG. 5 is a graph illustrating a transmittance when a voltage is appliedto the transflective LCD device of FIG. 4 in the related art LCD device;

FIG. 6 is a schematic cross-sectional view of a transflective LCD deviceillustrating an operation of such devices according to a firstembodiment of the present invention;

FIG. 7 shows a positional relationship among elements of thetransflective LCD device shown in FIG. 6;

FIGS. 8A and 8B are graphs illustrating a reflectance in the reflectiveportion and a transmittance in the transmissive portion of thetransflective LCD device, respectively;

FIG. 9 is a schematic cross-sectional view of the transflective LCDdevice according to a second embodiment;

FIG. 10 shows a positional relationship of the elements of thetransflective LCD device shown in FIG. 9; and

FIGS. 11A and 11B are graphs respectively illustrating reflectance andtransmittance with variations in applied voltages according to thesecond embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

FIG. 6, a schematic cross-sectional view of a transflective LCD device100 illustrating an operation of such devices according to a firstembodiment of the present invention. For convenience, the color filters(the reference numeral 17 of FIG. 1) are not shown in FIG. 6 becausethey do not affect the polarization state of light. Also, some ofexplanation will be omitted if not necessary.

As shown in FIG. 6, the transflective LCD device 100 includes upper andlower substrates 143 and 154 and an interposed liquid crystal layer 123that has an optical retardation of λ/4. A common electrode 147 is formedon the upper substrate 143. On the other surface of the upper substrate143, an upper quarter wave plate (QWP) 145 (often referred to as anupper retardation film), which has a phase difference λ/4, and an upperpolarizer 155 are successively formed thereon.

A transparent electrode 195 is formed on the lower substrate 154. Apassivation layer 193 and a reflective electrode 191 b are successivelyformed on the transparent electrode 195. The reflective electrode 191 band the transparent electrode 195 act together as a pixel electrode. Onthe other surface of the lower substrate 154, a lower quarter wave plate(QWP) 142 (referred to as a lower retardation film) and a lowerpolarizer 152 are successively formed thereon. Moreover, a backlightdevice 161 is arranged to be adjacent to the lower polarizer 152.

In the first embodiment, the liquid crystal layer 123 includes ahomogeneous liquid crystal. Therefore, the liquid crystal molecules, asaforementioned, are vertically arranged when the voltage is applied, andthus the transflective LCD device displays the dark state. However, themolecules close to the pair of substrates, as described in FIG. 3, arenot arranged to be perpendicular to the substrates. This causes theliquid crystal layer to have an optical retardation value. As a result,the light passing through the liquid crystal layer has the opticalretardation so that some portion of the light leaks from the panel dueto a wavelength dispersion.

Referring back to FIG. 5, the transmittance of the light passing throughthe liquid crystal layer is measured (i.e., T=0.038) when the voltage isapplied. Then the optical retardation “Δn·d” is calculated usingequation (4). The optical retardation of the liquid crystal can becompensated by adjusting a thickness of the liquid crystal layer.

Referring to FIG. 6, for instance, suppose that the optical retardationvalue of the upper retardation film is 140 nm (at Δn=0.0028, d=50micrometers). Further, suppose that the liquid crystal layer is ZGS-5063(Δn=0.067), which is conventionally used. If a voltage is applied to theliquid crystal layer, an optical retardation value of the light passingthrough the liquid crystal layer should be ideally zero. However, due tothe molecules arranged close to the substrates, the light passingthrough the liquid crystal has a non-zero optical retardation. Thus,when the transmittance is 0.038 (T=0.038) as shown in FIG. 5, the liquidcrystal layer has the optical retardation of 34 nm (Δn·d=34 nm) usingequation (4).

From the equation of Δn·d=34 nm, “d” of the retardation film is about 12micrometers (i.e., d=12 micrometers) when the birefringence “Δn” is0.0028. Thus, the retardation film is thicker by 12 micrometers (i.e.,α=12 micrometers) than it could be (i.e., d₃+α, shown in FIG. 6).Moreover, “d” of the liquid crystal layer is about 0.5 micrometers(i.e., d=0.5 micrometers) from the equation of Δn·d=34 nm when Δn=0.067.Thus, the liquid crystal layer is thicker by 0.5 micrometers (i.e.,β=0.5 micrometers) than it could be (i.e., d₄+β, shown in FIG. 6).

Each thickness of the liquid crystal layer and the retardation film maybe described by the following TABLE 1.

TABLE 1 Reflective Transmissive V_(off) Upper retardation film λ/4 + Δn· d λ/4 + Δn · d Liquid crystal layer λ/4 + Δn · d λ/2 + Δn · d Lowerretardation film Don't care λ/4 V_(on) Upper retardation film λ/4 + Δn ·d λ/4 + Δn · d Liquid crystal layer Δn · d Δn · d Lower retardation filmDon't care λ/4

Accordingly, as shown in TABLE 1, the transflective LCD device in thepresent invention can have a completely polarized light in thereflective and transmissive modes by controlling the thickness of theliquid crystal layer 123 and of the upper retardation film 145.

When a voltage is not applied to the liquid crystal layer 123, the lightpassing through the reflective portion of the liquid crystal layer 123has an optical retardation of λ/2. On the other hand, the light passingthrough the transmissive portion of the liquid crystal layer 123 has theoptical retardation of λ/2. Conversely, when a voltage is applied to theliquid crystal layer 123, both the light passing through the reflectiveand the transmissive portions of the liquid crystal layer 123 has a zerooptical retardation.

Accordingly, the complete dark state of the transflective LCD device canbe achieved in both transmissive and reflective modes. Although theoptical retardation is λ/2 in the liquid crystal layer when a voltage isnot applied, the optical retardation of λ/4 is acceptable forsimplifying the fabrication process step.

In addition to the method of adjusting the thickness of the liquidcrystal layer and of the retardation film, the transflective LCD devicemay be designed by using other ways. For example, a slow axis of theupper QWP 145 is designed to be perpendicular to that of the lower QWP142. Alternatively, the slow axis of the lower QWP 142 is parallel withan orientation direction of the liquid crystal layer, as shown in FIG.7. Moreover, a transmissive axis of the upper polarizer 155 isperpendicular to that of the lower polarizer 152. Therefore, theresidual optical retardation of the liquid crystal layer in the voltageof on-state is offset by these perpendicular structures. A light leakageis thus prevented when the transflective LCD device displays the darkstate.

FIGS. 8A and 8B are graphs illustrating reflectance in the reflectiveportion and transmittance in the transmissive portion of thetransflective LCD device, respectively. As shown, when a voltage isapplied (V_(on)=5V), a light leakage does not occur in both thereflective and transmissive portions of the transflective LCD device. Onthe other hand, when a voltage is not applied (V_(off)=0V), both thereflectance and the transmittance are about 0.3. These are almost equalto each other.

A contrast ratio is relatively more affected by the brightness of theLCD device in the dark state. According to the present invention, sincethe complete dark state can be achieved when a voltage is applied, thetransflective LCD device has a high contrast ratio.

A second embodiment of the present invention will be explainedhereinafter referring to accompanying drawings. In the secondembodiment, thickness of both the upper and lower retardation films isadjusted for compensating an optical retardation of the transmissive andreflective portions. The thickness of the liquid crystal layer is alsoadjusted.

FIG. 9 is a schematic cross-sectional view of a transflective LCD deviceaccording to the second embodiment. For convenience, the color filters(the reference numeral 17 of FIG. 1) are not shown in FIG. 9 becausethey do not affect the polarization state of light.

As shown in FIG. 9, the transflective LCD device 200 includes upper andlower substrates 217 and 229 and an interposed liquid crystal layer 221.The upper substrate 217 includes a common electrode 219 on the surfacefacing into the lower substrate 229. On the other surface of the uppersubstrate 217, an upper quarter wave plate (QWP) 215, an upper half waveplate (HWP) 213, and an upper polarizer 211 are successively formedthereon. The upper HWP 213 has an optical retardation of 270 nm (λ/2; atλ=550 nm) and the upper QWP 215 has an optical retardation of 140 nm(λ/4; at λ=550 nm). The upper HWP 213 and the upper QWP 215 act as firstand second upper retardation films, respectively.

Still referring to FIG. 9, the lower substrate 229 includes atransparent electrode 227 on the surface facing into the upper substrate217. A passivation layer 225 and a reflective electrode 223 b aresuccessively formed on the transparent electrode 227. A transmittinghole 223 a penetrating both the reflective electrode 223 b and thepassivation layer 225 is formed in the central part of both thereflective electrode 223 b and the passivation layer 225. The reflectiveelectrode 223 b and the transparent electrode 227 act together as apixel electrode.

On the other surface of the lower substrate 229, a lower quarter waveplate (QWP) 231, a lower half wave plate (HWP) 233, and a lowerpolarizer 237 are successively formed thereon. The lower QWP 231 has anoptical retardation of 140 nm (λ/4; at λ=550 nm) and the lower HWP 233has an optical retardation of 270 nm (λ/2; at λ=550 nm). The lower HWP233 and the lower QWP 231 act as first and second lower retardationfilms, respectively. The lower polarizer 237 has a transmissive axisthat is perpendicular to a transmissive axis of the upper polarizer 211.Moreover, a backlight device 235 is arranged adjacent to the lowerpolarizer 237.

FIG. 10 shows a positional relationship of the transflective LCD deviceof FIG. 9 according to the second embodiment. As shown in FIG. 10, thetransmissive axis of the upper polarizer 211 is perpendicular to that ofthe lower polarizer 237. A slow axis of the upper HWP 213 isperpendicular to that of the lower HWP 233 while a slow axis of theupper QWP 215 is perpendicular to that of the lower QWP 231. Moreover,an orientation direction of the liquid crystal layer is parallel to theslow axis of the lower QWP 231. The optical axes of the lower partelements is perpendicular to those of their mutual upper part elements,resulting in that optical compensation effect of the light passingaforementioned transflective LCD device is achieved with highefficiency.

The reason of forming two retardation films (QWP and HWP) on eachsubstrate is to expand an optical compensation effect through thebroad-wavelength band of the light. In other words, the slow axis of thequarter wave plate should be disposed at an angle of 45° from thetransmissive axis of the polarizer in order to convert the linearlypolarized light to the circularly polarized light. By achieving theangle of 45° between the transmissive axis of the polarizer and the slowaxis of the retardation film, the linearly polarized light that haspassed through the polarizer is converted into the circularly polarizedlight by the phase difference of λ/4.

Moreover, the optical retardation of an object is depending on thewavelengths of the incident light, and a general retardation film isdesigned focusing on the wavelength of 550 nm that is the centralportion of the visible light (380 nm to 780 nm). Thus, the opticalretardation of the general retardation film is λ/4, (i.e., 140 nm).Therefore, the retardation film does not properly serve to produce thecircularly polarized light in the other wavelengths, resulting in theelliptically polarized light. In order to solve this problem, tworetardation films are used in both upper substrate and lower substrate.This structure shown in FIG. 10 enables to control all wavelengths ofthe visible light in accordance with the angle “θ”.

An operating principle of the above-mentioned transflective LCD devicewill be explained hereinafter.

As incident light passes through the polarizer, the light is convertedinto the linearly polarized light. When the linearly polarized lightfaces the retardation film having an optical retardation of 270 nm, adirection of the linearly polarized light is changed by the optical axisof the retardation film. However, the retardation film act as a halfwave plate (λ/2) for a designed wavelength such as λ=550 nm, so that theretardation film produces the elliptically polarized light in the otherwavelengths.

Thereafter, the light results in having an angle of 45° from the slowaxis of the QWP such that the linearly polarized light is converted intothe circularly polarized light. Before the light enters the QWP, the HWPcompensates the light for its optical retardation, and thus allwavelengths of the linearly polarized light are nearly converted intothe circularly polarized light.

However, the compensation effect of the retardation film as discussedabove has some limitations as follows.

First of all, since a homogeneous liquid crystal is not completelyarranged in the vertical direction when a voltage is applied, a phasedifference caused in the liquid crystal layer is not completelycompensated. In addition, since there is a phase difference in thedifferent cell gaps, (i.e., reflective portion and transmissiveportion), it is difficult to precisely compensate the phase differencescaused in the reflective and transmissive portions at the same time.

Accordingly, in order to compensate the phase differences caused both inthe reflective and transmissive potions, not only the thickness of theliquid crystal layer but also the thickness of the upper and lower QWPsare adjusted at the same time. Thus, the phase of the light iscompensated accordingly. In other words, a phase value occurring in thedifferent cell gaps of the liquid crystal layer is greater in thetransmissive portion than in the reflective portion.

At this point, suppose that a phase difference of the reflective portionis γ and a phase difference of the transmissive portion is ω. If thephase difference δ between the transmissive and reflective porions isω−γ (i.e., δ=ω−γ), then the phase difference of the transmissive portionis ω=δ+γ. Therefore, the cell gaps of the transmissive and reflectiveportions is written as following equations:d_((λ/4))+d_((γ))  (5): cell gap of reflective portiond_((λ/2))+d_((ω=δ+γ))  (6): cell gap of transmissive portion

Here, an orientation direction of the liquid crystal layer has an angleof 90° from the slow axis of the upper QWP and an angle of 0° from theslow axis of the lower QWP. Thus, an offset occurs between the liquidcrystal layer and the upper retardation film, while reinforcement occursbetween the liquid crystal layer and the lower retardation film. Here,analysis of the state of polarization will be in terms of a formaccording to the Jones matrix method, in which two components, of theelectric field, perpendicular to a direction of movement of light areexpressed as a vector, and the transmission medium is expressed as a 2×2matrix:

$\begin{matrix}\begin{pmatrix}{\mathbb{e}}^{{\mathbb{i}ɛ}_{x}} & 0 \\0 & {\mathbb{e}}^{{\mathbb{i}ɛ}_{y}}\end{pmatrix} & (6)\end{matrix}$

wherein ε_(x) and ε_(y) denote optical retardations of x-axis and ofy-axis, respectively, in orthogonal coordinates system. Here, the properoptical retardation is represented by Δε=ε_(y)−ε_(x). Therefore, if theslow axis of the upper QWP (140 nm) is perpendicular to the optical axisof the liquid crystal layer, the slow axis of the upper QWP is thex-axis and the orientation direction is the y-axis. Δε of the upper QWPbecomes a negative value (∵ε_(y)<ε_(x)), and Δε of the liquid crystallayer becomes a positive value (∵ε_(y)>ε_(x)), and then the opticalretardation Δε is offset by each other.

However, since the optical axis of the liquid crystal layer is parallelto the slow axis of the lower QWP, the optical axis of the liquidcrystal layer and the slow axis of the lower QWP can altogether eitherbe x-axis or y-axis in orthogonal coordinates system. Therefore, theoptical retardation Δε of both the liquid crystal layer and the lowerQWP has either positive or negative value, and then the opticalretardation Δε is reinforced.

From the equations (5) and (6), the thickness of the upper and lower QWPis adjusted, resulting in that the optical retardation of the upper QWPis 140 nm+γ and the optical retardation of the lower QWP is 140 nm−γ.Thus, the optical retardation is offset by each other.

The thickness of the liquid crystal layer and of the retardation filmscan be described by the following TABLE 2.

TABLE 2 Reflective Transmissive V_(off) Upper retardation HWP 270 nm 270nm films QWP 140 nm + γ 140 nm + γ Liquid crystal layer 140 nm + γ 270nm + γ + δ Lower retardation QWP Don't care 140 nm − δ films HWP 270 nmV_(on) Upper retardation HWP 270 nm 270 nm films QWP 140 nm + γ 140 nm +γ Liquid crystal layer γ γ + δ Lower retardation QWP Don't care 140 nm −δ films HWP 270 nm

For more detailed explanation, as shown in TABLE 2 and FIGS. 9 and 10,the state of complete polarization of the light can be achieved byadjusting the thickness of both the liquid crystal layer 221 and theupper and lower QWPs 215 and 231. When a voltage is not applied, thepart of optical retardation (γ) caused by the liquid crystal layer 221in the reflective portion is offset by the upper QWP 215. The, part ofoptical retardation (γ+δ) caused by the liquid crystal layer 221 in thetransmissive portion is also offset by both the upper and lower QWPs 215and 231. Thus, every components of the light passing through both thereflective and transmissive portions has the optical retardation of λ/2(at λ=550 nm).

On the other hand, when the voltage is applied, the optical retardation(γ) caused by the liquid crystal layer 221 in the reflective portion isoffset by the upper QWP 215, and thus the light passing through thereflective portion has the optical retardation of zero. Moreover, theoptical retardation (γ+δ) caused by the liquid crystal layer 221 in thetransmissive portion is also offset by both the upper and lower QWPs 215and 231, and thus the light passing through the transmissive portion hasthe optical retardation of zero. Accordingly, the dark state of thetransflective LCD device can be achieved in both transmissive mode andreflective mode as described before.

After fabricating the LCD panel that has above-mentioned structure, thetransmittance is measured by a simulator such as an LCD master. Theresults are shown in FIGS. 11A and 11B. At this time, the liquid crystallayer is ZGS-5063 (Δn=0.067) that is conventionally used. A thickness ofthe liquid crystal layer is 2.3 micrometers. A phase difference of thereflective portion is γ=17 nm. A phase difference of the transmissiveportion is ω=34.8 nm. Thus, the phase difference in between thereflective and transmissive portions is δ□18 nm from the above-mentionedequation ω=γ+δ.

FIGS. 11A and 11B are graphs illustrating the reflectance and thetransmittance, respectively, depending on the applied voltage.

Referring to FIG. 11A that shows the reflectance in the reflective mode,because of the compensation effect of both the upper QWP and the liquidcrystal layer, the reflectance is about 0.27 when the voltage is notapplied (i.e., V_(off)=0V). However, when the voltage is applied (i.e.,V_(on)=5V), the reflectance is zero. Thus, the light is not reflectedcompletely when the TFT is turned ON.

Referring to FIG. 11B that shows the transmittance in the transmissivemode, because of the compensation effect of the upper and lower QWPs andof the liquid crystal layer, the transmittance is about 0.32 when thevoltage is not applied (i.e., V_(off)=0V). When the voltage is applied(i.e., V_(on)=5V), the transmittance is zero.

Therefore, the complete dark state of the LCD device can be achieved inboth tansmissive and reflective modes when the voltage is applied. Fromthese results, the high contrast ratio can be achieved in thetransflective LCD device.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention covers the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. A transflective liquid crystal display, comprising: upper and lowersubstrates facing into and spaced apart from each other, wherein theupper and lower substrates include a plurality of pixel regions thatdisplay images; an upper quarter wave plate (QWP) on the uppersubstrate; an upper polarizer on the upper quarter wave plate; a lowerquarter wave plate (QWP) below the lower substrate; a lower polarizerbelow the lower quarter wave plate; a backlight device arranged to beadjacent to the lower polarizer; a liquid crystal layer interposedbetween the upper and lower substrates; a transparent common electrodeon a surface of the upper substrate facing into the lower substrate; anupper alignment layer between the transparent common electrode and theliquid crystal layer; a pixel electrode over the lower substrate,wherein the pixel electrode corresponds to each pixel region, and thepixel electrode is divided into transparent and reflective portions; anda lower alignment layer between the pixel electrode and the liquidcrystal layer; wherein a transmissive axis of the upper polarizer isperpendicular to a transmissive axis of the lower polarizer, a slow axisof the upper QWP is perpendicular to a slow axis of the lower QWP, theslow axis of the upper QWP forms an angle of 45° with the transmissiveaxis of the upper polarizer, an optical retardation of the upper QWP isλ/4+α, and an optical retardation of the liquid crystal layer is λ/4+αin the reflective portion so that the upper QWP has the same opticalretardation as the liquid crystal layer in the reflective portion,ranges from zero to 100 nm, and the slow axis of the lower QWP isparallel to an orientation direction of the liquid crystal displaylayer, and wherein the liquid crystal layer has an adjusted thickness,and wherein the adjusted thickness is d+d1, where d is a normalthickness of the liquid crystal layer and d1 is calculated using thefollowing equation,${T = {{Sin}^{2}2\phi\;{{Sin}^{2}\left\lbrack \frac{{\pi \cdot \Delta}\;{n \cdot d_{1}}}{\lambda} \right\rbrack}}},$ where T is a value of transmittance when a maximum operation voltage isapplied, φ is an angle between an optical axis of the liquid crystallayer and a transmissive axis of the polarizer, Δn is a birefringence ofthe liquid crystal layer.
 2. The transflective liquid crystal displayaccording to claim 1, wherein an optical retardation of the liquidcrystal layer is different between transmissive and reflective portions,the optical retardation is λ/2+β in the transmissive portion, and βranges from zero to 100 nm.
 3. The transflective liquid crystal displayaccording to claim 2, wherein an optimum value of β for adjusting theoptical retardation ranges from zero to 50 nm.